ALBERT

Description: Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C: N: P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C: N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C: N ratio ranged uniformly around 7.5. N: P ratios 〈13 indicated N limitation, while N: P ratios 〉22 indicated P limitation. Under P limitation, the C: P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C: N: P = 119 : 17 : 1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106 : 16 : 1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.

Description: According to Connell�s intermediate disturbance hypothesis (IDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different frequencies and intensities were imposed on natural plankton communities in controlled field experiments. These disturbances consisted of an artificial deepening of the mixed layer, leading to the dilution of epilimnetic populations and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differences in the experimental mixing depth (150 and 225% of the original epilimnion depth). Investigation focused on the effect that disturbances had on the diversity of natural phytoplankton communities. Additionally, we were interested in determining the effect of grazing by zooplankton. The results of the field experiments show for the first time the applicability of the IDH to phytoplankton within complete planktonic communities. Diversity showed a clear maximum at the intermediate disturbance interval of 6 d. Similarly, species number peaked at intermediate interval length (6-10 d).

Description: We deployed CO2 and O2 sensors on the U.S. continental shelf off Cape Hatteras, North Carolina, during late summer 1994. A continuous 32‐d gas record was obtained at 20 m in 25 m of water, below the thermocline for most of the period. Analysis of the correlation between CO2 and O2 indicates that biological and advective processes dominated the gas variability, with small or insignificant fluxes due to air–sea exchange, vertical eddy diffusion, and carbonate dissolution or formation. The observed O2 : CO2 correlation was 1.39, within the range predicted for the photosynthetic quotient. Photosynthesis and respiration appeared to be tightly coupled, resulting in no net community production in these waters during the late summer. It is evident from these results that the combination of mooring‐based CO2 and O2 measurements will be a powerful tool for studying the marine carbon cycle.

Description: The effect of variable concentrations of dissolved molecular carbon dioxide, [CO2,aq], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient-replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO2,aq]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO2 concentration below which a further decrease in [CO2,aq] led to large changes in C:N:P ratios. Significant CO2-related changes in elemental ratios were observed at [CO2,aq] 〈 10 mu mol kg-l and correlated with a CO2-dependent decrease in growth rate. At [CO2,aq] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern far CO2-related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO2,aq] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in Skeletonema costatum, was of a similar magnitude as CO2-related variation. In this species, the CO2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon-rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO2-related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle